In computer graphics, a sampled image is created from input geometry or mathematical computations that typically represent a scene. The sampled two-dimensional (2D) image is represented by an array of discrete units referred to as pixels. The pixels are generally arranged along two perpendicular axes corresponding to the horizontal and vertical axes of the image. The color or monochrome values of the pixels are determined by calculated sample values, typically derived from scene data, lighting data, or other input data. The image represented by the array of pixels is typically transferred to a visual medium, such as by being printed onto paper or film, or displayed upon a computer display device. The number of pixels and number of colors or values used in rendering a graphics image limit, to some extent, the visual qualities and characteristics of the viewed image, for example, the image definition, smoothness, and detail. Consequently, a great amount of effort has been devoted to developing sophisticated graphics processing and rendering techniques for higher image quality.
In addition to high-quality static images, high-quality animation depicting visual effects of motion, is also desirable. The illusion of motion is created by quickly displaying related images in a sequence of image frames in which an object appears in different positions. Although the motion of the object may appear to be continuous, each frame of the animation is a separate image that is displayed momentarily. Thus, the quality of each frame will affect the quality of the animation or the illusion of motion. Depending upon the rendering algorithm, representing motion may result in the creation of motion artifacts. The more noticeable effects include static edge-effects such as “staircasing,” as well as “crawling” and “popping” of pixels on an edge of a moving object having a value that contrasts against a background having a different value.
For example, consider an object having a straight edge and of a first color which appears to be moving across a background of a second color. As the edge of the object moves across a pixel, there must be a determination as to when the color of the pixel changes from the color of the background to the color of the object. If a single sample location within the pixel is taken to determine its color, then, when the edge of the object passes the single sampling location, the color of the pixel is changed. The location of a single sample location is typically located at the center of the pixel, and thus, the value of the pixel is determined by the value calculated for the center of the pixel. As a result of a single sample point determination, the edge of an object may extend well into the region represented by the pixel, but because the edge has not reached the center (i.e., the sample point) of the pixel, the pixel may continue to have the color of the background. As a result, when multiple images or frames are displayed in sequence to provide the illusion of objects in motion, pixels along the edge of an object may “pop” from one color or value to another. This effect can be distracting for a viewer. The relative motion of an object with respect to the orientation of the pixels of the image may be such that pixels along the edge of the object pop values in a manner and with a regularity that creates a visual effect of the edge of the object “crawling.”
There have been many different approaches to addressing the issue of aliasing. One such approach is to increase the resolution, or the number of pixels used to represent an image. Although the available resolution of computer graphics displays and computer printing devices have reduced aliasing, there are practical limitations on the manufacture of displays and other devices used for forming or recording an image which will limit the maximum available resolution. Moreover, no matter how high a (finite) resolution is used to represent a graphics image, so long as the value of each pixel is only approximated from calculated values for discrete, sample points, within the pixel, the image is subject to some degree of aliasing, and a sequence of such images is subject to motion artifacts.
In another approach, aliasing can be “antialiased” to some degree by calculating multiple number of values in determining a final value for each pixel. That is, multi-sampling systems for antialiased rendering use sample values from multiple samples taken from a pixel region in determining the value of the respective pixel. For example, when multi-sampling is applied to the previous example, as the edge of the object passes the first of the sample locations in a pixel region, the pixel is given a value that is a compromise between the value of the object and the disparate value of the background. Where the size, shape and motion of the object result in the object gradually and completely covering the pixel, the value of the pixel will change each time the object covers another sample location until all of the sample points share the color of the object, at which time the pixel takes on the color of the object.
One approach to implementing the previously described antialiasing technique has been to shift the input geometry of the entire three-dimensional (3D) environment a sub-pixel distance for each of the sub-pixel sample locations and then repeat the entire rendering or rasterization process. In effect, the rendering process, from generating scene geometry through rasterizing the scene, is repeated for the calculation of each of the multiple samples. Although the resulting image is antialiased, this approach consumes considerable processing resources of a graphics processing system because multiple passes through the process of generating the scene geometry are needed. The time necessary to generate the multiple frames is a linear function of the number of desired samples, and as a result, no economy of scale performance gains would typically be realized. Moreover, the generating the geometry of the 3D environment multiple times necessitates either application driven generation of the frames, or an arbitrary amount of buffering within the graphics processing system to store the resulting data for an arbitrarily complex scene.
Therefore, there is a need for an alternative system and method for rendering an image from a representation of a scene, while reducing aliasing, such as motion artifacts and edge effects.